In color printing using digital printers, it is common to require occasional “calibration” of the printer. Generally this is done by causing the printer to output sheets bearing a series of “test patches,” each patch representing a desired color. The test patches are then read by a spectrophotometer or similar image sensor, and the actual reflectance values of the patches are compared to the colors of the desired patches. In a high-speed, production context, it is known to provide image sensors immediately downstream of a printing apparatus for various purposes, as shown in U.S. Pat. Nos. 5,488,458; 6,324,353; and 6,684,035.
In practical operation, an image sensor used in combination with a printer must itself be calibrated occasionally. Sensor calibration usually involves exposing to the sensor a surface of known predetermined optical properties, such as a predetermined blackness or whiteness, and then adjusting the outputs of the image sensor accordingly. U.S. Pat. No. 6,198,536 shows a sheet scanner in which calibration targets can be slid underneath a spectrophotometer: a user manually slides the desired black or white calibration “backer” underneath the spectrophotometer as needed.
The present disclosure, in various embodiments, is directed to a system useful in calibrating image sensors, whether as part of a testing station downstream of a printing apparatus, within a scanner for recording hard-copy images, or for any other purpose.